1. Field Of The Invention
The present invention relates to a tuning circuit with an amplitude attenuation function and an integrated circuit for radio communication apparatuses.
2. Description of the Related Art
FIG. 16 shows a typical example of a tuning circuit and an AGC (Automatic Gain Control) circuit in, for example, ASK (Amplitude Shift Keying) communication. The tuning circuit comprises an LC parallel resonance circuit where one ends of a coil (inductance) L1 and a capacitor (capacitance) C1 are connected to a reference voltage Vref. The AGC circuit is made up of a variable gain amplifier, a rectifier (REC), and a comparator (COMP). The variable gain amplifier adjusts the amplitude of an AC (alternate current) signal from the tuning circuit and outputs through its output terminal OUT. This output terminal OUT is connected to an amplifier, a detection circuit, and a waveform shaping circuit, which process the AC signal adjusted in amplitude. Refer to, e.g., Japanese Patent Application Laid-open Publication No. 10-23084.
The gain for the AC (alternate current) signal of the variable gain amplifier is determined in the rectifier and comparator. Specifically, after the rectifier produces a DC signal by smoothing the amplitude of the AC signal, the comparator compares this DC signal with a reference voltage VAGC. As a result of this comparing, when the amplitude of the AC signal is excessive, the comparator feeds back its output for decreasing the gain to the amplifier. Thus, the amplitude of the AC signal, which would otherwise be excessive, is controlled to be suppressed always being maintained at a given constant level.
Such a tuning circuit and an AGC circuit are used in, for example, the receiver of a remote control system. Such remote control systems have various applications such as the open and close and locking of doors of cars, houses and the like, and the start and stop of car engines.
When realizing the AGC function such as automatic control to reduce the amplitude of the AC signal when it is large, using an analog control system comprising the variable gain amplifier and the rectifier mentioned above renders power consumption large (about 1 μA in terms of electrical current). If this circuit large in power consumption is used in, e.g., a battery-operated-type receiver of a remote control system, the battery will be used up earlier.